Антенны, СВЧ / OC / Broadband microstrip antennas

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9.2 Planar Rectangular and Square Monopole Antennas

A planar rectangular monopole antenna can be thought of as a variation of the RMSA, in which the horizontal ground plane is considered to be located at infinity. The following discussions bring out this analogy.

9.2.1 RMSA Suspended in Air with Orthogonal Ground Plane

The side and the front views of a rectangular radiating patch with L = W = 12 cm made of a copper plate of thickness 0.1 cm with two orthogonal ground planes are shown in Figure 9.2(a, b). The patch is fed with a 50-V SMA connector of probe length p through a fixed ground plane and the orthogonal ground plane is moveable. For the moveable ground plane spacing h = 3 cm from the radiating patch and the probe length p = 0.4 cm, the measured input impedance and VSWR plots are shown in Figure 9.2(c, d). Multiple loops occur due to the excitation of various higher order modes of RMSA. The impedance plot shows less inductive shift due to the smaller value of the feed probe length p, so the value of p is increased to 1 cm to shift the impedance plot in the clockwise direction. The results are summarized in Table 9.1. The measured BW for VSWR ≤ 2 is from 858 MHz to 988 MHz.

For analysis purpose, this structure can be thought of as an MSA on an air dielectric with an additional perpendicular ground plane. The analytical methods valid for MSAs could be applied to this configuration with suitable modifications.

The formula for RMSA is used to calculate the resonance frequency of this antenna. Since the dielectric medium for all the cases under consideration is air, the effective dielectric constant ee is equal to 1. The theoretical resonance frequency for the fundamental mode can be calculated using (2.9).

where:

L e = effective resonant length;

c = velocity of light in free space.

For two orthogonal ground planes, L e . L + DL + p, because the extension of length DL due to the fringing fields is applicable only for one side, and on the other side, it is restricted to p due to the orthogonal ground

Figure 9.2 (a) Side and (b) front views of modified RMSA with orthogonal ground planes. Measured (c) input impedance and (d) VSWR plots.

plane. For a large width of the patch (W /h > 10) with er = 1, DL is approximately equal to h. The theoretical resonance frequency, calculated using (9.1) for p = 1.0 cm, is 937 MHz, which is close to the measured center frequency of 923 MHz.

Next, the effect of increasing h on BW of the antenna is considered. For different values of h with p = 1.0 cm, the measured lower and upper frequencies ( f L and f H ) corresponding to VSWR = 2 are given in Table 9.1. With an increase in h from 3 cm to infinity (∞), the percentage BW

the input impedance and VSWR plots are nearly the same. Therefore, for a large h tending to infinity, the MSA configuration reduces to that of a planar monopole antenna. In this case, the approximate lower frequency corresponding to VSWR = 2 can be determined by using the monopole antenna concept described in Section 9.2.2.

9.2.2Calculation of the Lower Frequency of the Planar Monopole Antennas

For a planar monopole antenna, the lower frequency corresponding to VSWR = 2 can be approximately calculated by equating its area (in this case, a rectangular disc monopole) to that of an equivalent cylindrical monopole antenna of same height L and equivalent radius r, as described below [3, 4]:

The input impedance of a l /4 monopole antenna is half of that of the l /2 dipole antenna. Thus, the input impedance of an infinitesimally thin monopole antenna is 36.5 + j 21.25V, which is inductive. The real input impedance is obtained when a slightly smaller length of the monopole

Equation (9.7) does not account for the effect of the probe length p, which increases the total length of the antenna thereby reducing the frequency. Accordingly, this equation is modified to

where L , r, and p are in centimeters.

The theoretical frequency of 483 MHz for h = ∞ (monopole antenna) obtained using (9.8) is close to the measured f L of 501 MHz. For h = 18 cm, the theoretical frequencies obtained using the MSA and monopole concepts are very close to each other.

Thus, an interesting transition in antenna characteristics (with respect to resonance frequency) is observed, as the ground plane spacing h is increased. For a smaller h, the measured resonance frequency is close to the theoretical frequency determined by the expressions applicable to the MSA and for a larger h, it is close to the frequency obtained using expressions for a monopole antenna.

9.2.3Effect of Various Parameters of Planar Rectangular Monopole Antennas

The previous section observed that the planar rectangular monopole (RM) antenna yields a large impedance BW [3, 4]. The BW of these antennas depends mainly on the width W of the plate, the diameter d of the feeding probe, and the length of the probe p. The SMA connector is generally used above 1 GHz for feeding the antenna, and hence, d is kept fixed at 0.12 cm. When the height L of the monopole antenna increases (or decreases), the lower edge frequency decreases (or increases), which is quite obvious. Therefore, the effects of width W and probe length p on the performance of the antenna are described.

The RM and square monopole (SM) antennas are analyzed using IE3D software [6]. Initially, the effect of p is described by keeping L = W = 4.5 cm. The input impedance and VSWR plots for two values of p are shown in Figure 9.4(a, b). As p is increased from 0.05 cm to 0.2 cm, the input impedance plot shifts up in the clockwise direction. This is because with an increase in p, the probe inductance increases and therefore the input impedance becomes more inductive. A broad BW of 1,335 MHz (68%) is obtained for p = 0.2 cm, compared to the BW of 668 MHz (40%) for p = 0.05 cm.

Figure 9.4 (a) Input impedance and (b) VSWR plots of RM antenna for two values of p [( —— ) 0.05 cm and ( - - - ) 0.2 cm]; (c) input impedance and (d) VSWR plots with L = 4.5 cm and p = 0.2 cm for two values of W [( - - - ) 4.5 cm and ( —— ) 3.5 cm].

Next, the effect of W is described by keeping other parameters fixed. The input impedance and VSWR plots for two values of W (4.5 cm and 3.5 cm) with L = 4.5 cm and p = 0.2 cm are shown in Figure 9.4(c, d). With a decrease in W, the size of the loop in the impedance plot increases and the plot shifts toward the right in the Smith chart. Since the lengths of these RMs are kept fixed at 4.5 cm, the lower band-edge frequency remains almost unchanged; this is predominantly determined by the length L of the planar monopole antenna. As W decreases from 4.5 cm to 3.5 cm, the BW increases from 1,335 MHz to 1,715 MHz due to an increase in loop size.